The present invention relates to a pneumatic tire, more particularly to an improved belt structure for reinforcing the tread portion.
In the radial tires provided with a tread reinforcing breaker, which are designed for high speed running in particular, so called edge protectors or edge bands are usually provided on the radially outside of the breaker edges to prevent the breaker edges from being risen by a centrifugal force during high speed running.
The reinforcing cords which are widely used in such a band structure, are organic fiber cords having a conventional double-twist structure in which a cord is formed by twisting several strands together (last twist), each strand formed by twisting organic fibers together (primary twist) in the opposite direction to the last twist.
In a double-twist structure, an apparent diameter of an organic fiber cord becomes relatively large in comparison with the under mentioned a single-twist structure. Accordingly, even if the cords used are the same weight, a band made of double-twist structure cords becomes heavier than a band made of single-twist structure cords because the topping rubber is increased in the thickness and the total weight.
On the other hand, during making the tire, in other words, when the tire is not vulcanized and thus the cords are not yet heat set, the double-twist structure cords show a relatively large elongation at a relatively light load. An unvulcanized tire is subjected to a high inflation pressure during vulcanization and thus the band cords therein are stretched, and they are set after the vulcanization. The tension and stretch of the band cords which depend on the twist structure can be controlled by adjusting the winding diameter, winding tension, vulcanizing conditions and the like, but it requires skill and trials which are not cost effective.
It is therefore, an object of the present invention to provide a pneumatic tire in which, by employing band cords having a single-twist structure in which the twist coefficient, elongation %, residual tension, and cord count are arranged in a well-balanced manner, the band can display its maximum constricting force by minimal band cords and thereby a tire weight reduction and a cost reduction are possible without deteriorating the high speed durability, high speed steering stability and the like.
According to the present invention, a pneumatic tire comprises
a carcass extending between bead portions through a tread portion and sidewall portions,
a breaker disposed radially outside the carcass in the tread portion, and
a band disposed radially outside the breaker and made of one or more cords laid at an angle of not more than 5 degrees with respect to the tire circumferential direction, the above-mentioned one or more cords each being an organic fiber cord having a single-twist structure, a cord thickness D of from 500 to 3200 dtex, a twist coefficient T of from 150 to 750 which is defined by the product of the square root of the cord thickness D and the twist number N per 10 cm length of the cord, and an elongation under a tension of 2.7 g/dtex of from 4.0 to 8.0%,
a cord count of the above-mentioned one or more cords per a unit width of 1 cm of the band being in a range of from 5 to 25,
in a state of the tire which is not inflated and loaded with no tire load, a tensile force Ls of the band cord in the tire being in a range of from 4 to 25 N, and a band constricting index LI being in a range of from 50 to 250 N, the band constricting index LI defined as the total of the tensile forces Ls of said one or more cords per a unit width of 1 cm of the band.